Conventionally, the communication protocol of CAN has widely been employed in communication between multiple electronic control units (ECUs) mounted in a vehicle. A communication system with the CAN communication protocol is so configured that multiple ECUs are connected to a common CAN bus, and messages are transmitted and received by an ECU at the reception side obtaining a signal output from an ECU at the transmission side to the CAN bus.
In such a communication system, various proposals have been made for how to address any error occurring in communication.
For example, Japanese Patent Application Laid-Open Publication No. 2004-348274 has proposed a communication failure diagnostic device in which a provisional failure determination flag is set and a continued failure determination timer is actuated if any one of abnormal states of error warning, error passive or bus off is detected as a result of monitoring the state of the CAN bus, and thereafter, when a failure determination time elapses without an error active state being detected twice in succession, a failure confirmation flag indicating a confirmed failure is set so that a “limp home” function is started.
In an existing communication system employing the CAN communication protocol, if an error occurs in communication, a communication device that transmits a message is able to know the occurrence of the error only when the error occurs during transmission of the message transmitted by itself. If a malfunction or the like occurs in the communication system, therefore, it is necessary to inspect all the communication devices included in the communication system for the malfunction in order to inspect as to which message transmission/reception is subject to an error, which makes it difficult to identify the cause of malfunction. The communication failure diagnostic device according to Japanese Patent Application Laid-Open Publication No. 2004-348274 cannot solve such a problem.